4.3 exchange of gases in the lungs. learning outcomes students should understand the following: the...
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Learning outcomesStudents should understand the following:
The essential features of the alveolar epithelium as a surface over which gas exchange takes place.
The exchange of gases in the lungs.
Alveoli The site of gas exchange in mammals is the
epithelium of the alveoli.
Minute air-sacs 100 – 300m in diameter
There are about 300 alveoli in each human lung.
Each alveoli is lined with epithelial cells 0.05 m to 0.3 m thick.
Each alveolus is surrounded by a network of pulmonary capillaries.
Actually, each air sac is found to be a bundle of air sacs. Together, they are known as an alveolus.
We can look inside the alveolus to get some idea of why they are shaped the way they are.
The outside of the alveolus is covered with tiny blood vessels.
Oxygen makes its way to special air sacs.Inside an alveolus
oxygen (O2) gas passes
through here
This O2 is then able to dissolve in
a small moist lining.
epithelial lining of the
alveolus
A cross-section of an alveolus
The moist lining also stops the alveolus from drying and cracking. It lubricates the insides of the air bag.
After the oxygen dissolves it also diffuses.
O2 O2O2O2
cell lining of alveolus
cell lining of capillary
blood
D I F F U S I O N
The oxygen molecules must diffuse through both the lining of the alveolus and the lining of the blood capillary.
They are eventually picked up by red blood cells.
Oxygen diffusion into red blood cells
The blood now carries this oxygen to the cells of the body.
Right Lung
Left Lung
blood vessel
body cells
blood
O2
O2
Blood leaving the lungs
Laws of diffusionThe movement of the oxygen from the blood to the cells also follows the law of diffusion.
It is highly concentrated within the blood.
Meanwhile the concentration is low within the cell.
blood coming from the lungs
body cell
high concentration
low concentration
Therefore the oxygen passes into the body cells.
Alveoli adaptationsAlveoli have the following adaptations which make
them a really good surface for gas exchange:
Large surface area to volume ratio – speeds up the rate of exchange
Thin exchange surfaces – short diffusion pathway
Partially permeable – to allow selected materials to diffuse easily
Steep concentration gradient maintained by breathing and blood flow
Alveoli adaptationsThe walls of the alveoli are moist and oxygen
diffuses into this liquid before diffusing across the alveolar wall and into the blood.
This liquid contains a surfactant which reduces surface tension and prevents the air-sacs collapsing.
It is produced in babies from 7 months onwards which is why those born before this time may have breathing difficulties.
Artificial surfactants are now being used which can help prevent brain damage due to lack of oxygen in the first few weeks.